Metastable beta-titanium alloy

ABSTRACT

Metastable β-titanium alloy contains, in mass %: from 1.5 to 3.5 aluminum; from 4.5 to 8.0 molybdenum; from 1.0 to 3.5 vanadium; from 1.5 to 3.8 iron; titanium balance. This alloy combines high strength and ductility. This allows to use it for production of a wide range of critical parts including fastener components and different coil springs (e.g. in automobile industry).

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional application based on U.S. Ser. No. 10/496,493 filed on Jul. 21, 2005 entitled “Metastable β-Titanium Alloy” which is a § 371 filing of a PCT/RU02/00502 filed Nov. 18, 2002, which claims priority to Russian Federation Application No. 2001131383 filed Nov. 22, 2001.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the non-ferrous metallurgy, and more particularly to the development of new titanium-base alloys combining high strength and ductility properties using relatively low-cost alloying elements. The alloys of this invention can be applied in a wide range of products especially fasteners and different coil springs.

2. Background Art

Pure titanium is normally of a hexagonal (alpha—α) structure that transforms to a body-centered cubic (beta—β) when heated above 882° C. The addition of alloying elements to titanium influences this transition temperature. In many alloys, this results in beta being retained at room temperature. A material may thus be produced which contains both alpha and beta phases or, in some circumstances, a material which is wholly beta. The relative amounts of alpha and beta phases in any particular alloy affect tensile strength, ductility, creep properties, weldability and formability.

A metastable-beta titanium alloy retains an all-beta structure upon air-cooling of thin sections. A beta titanium alloy is termed “metastable” because the resultant beta phase is not truly stable—it can be aged to precipitate alpha for strengthening purposes. As used herein, the “metastable” terminology embraces a near-beta alloy which may also decompose to alpha plus beta upon aging.

One of the known titanium alloys is the alloy containing (mass %): 2-6 Al; 6-9 Mo; 1-3 V; 0.5-2 Cr; 0-1.5 Fe; Ti balance. Ref: USSR Inventor's Certificate No. 180351, Class C22C 14/00, 1966.

However, this alloy has insufficient ductility due to the high content of Al and the presence of Cr. Besides this alloy is rather expensive.

The other known titanium alloy contains (mass %): 4-6.3 Al; 4.5-5.9 V; 4.5-5.9 Mo; 2.0-3.6 Cr; 0.2-0.5 Fe; Ti balance. Ref: RF Patent No. 2169204, Class C22C 14/00, published 2001.

The said alloy as-heat treated has high strength properties in heavy section forgings, but its ductility is insufficient and so the alloy cannot be used for production of such parts as coil springs.

The most close to the claimed invention is the metastable β-titanium alloy containing (mass %): 4-5 Fe; 4-7 Mo; 1-2 Al; 02 up to 0.25; Ti balance. Ref: U.S. Pat. No. 5,294,267, Class C22C 14/00, published 1994. This alloy will be the prototype.

This alloy has high machinability, demands relatively low costs and is widely used for production of coil cylindrical springs in automotive industry.

However, the said alloy has low ductility properties, especially elongation, which reduces the application of this alloy and is of importance during manufacture of some types of coil springs and fastener components.

It is known that the beta phase in pure titanium is stable from approximately 882° C. (1620° F.) to the melting point of about 1688° C. (3040° F.). Where beta is the predominant phase in the microstructure of a titanium alloy, certain properties may be obtained depending upon the processing methodologies followed.

SUMMARY OF THE INVENTION

The object of this invention is to provide a titanium alloy with combination of high ductility and strength properties in as-heat treated condition, which can be produced using low cost alloying elements.

In accordance with the invention this is achieved by addition of vanadium to the metastable β-titanium alloy containing aluminum, molybdenum and iron at the following content of components (mass %):

Aluminum 1.5-3.5 Molybdenum 4.5-8.0 Vanadium 1.0-3.5 Iron 1.6-3.8 Titanium balance

The addition of 1.0-3.5% vanadium increases the alloy ductility as required.

To achieve high strength properties this alloy has higher content of aluminum than the prototype. The content of aluminum of less than 3.5% does not significantly influence on the alloy ductility. The contents of aluminum greater than 3.5% and iron greater than 3.8% increases the α-phase quantity, causes hardening and reduces ductility lower than desired. The lower content of iron (<4%) than in the prototype ensures greater phase stability during thermal cycles (deformation and heat treatment). The desired strength properties cannot be achieved with aluminum below 1.5%. The content of molybdenum below 4.5% and iron below 1.6% reduces β-phase quantity and does not lead to high strength of the as-heat treated alloy.

The increase in the content of such P-stabilizers as molybdenum and vanadium exceeding the specified limits reduces the alloy stability in hardened and aged conditions and increases the grain size during heat treatment, which significantly reduces the alloy ductility (β<4%; Ψ<7%).

Molybdenum is added as ferromolybdenum with 55-75% of molybdenum and iron balance.

Vanadium is added as ferrovanadium with 65-85% of vanadium and iron balance or Ti—Al—V system scrap.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

To study the properties of the alloy, ingots with the composition shown in Table 1 were melted in a vacuum arc furnace and 20 mm diameter bars were made from these ingots. The bars were heat treated under the following conditions: heating to temperature of 30° C. below beta transus temperature, water cooling, heating to 480° C. for 8 hours, air cooling. Then tensile specimens were tested according to ASTM E 8.

As used herein, the “beta transus temperature” refers to the temperature at which the titanium alloy will be transformed completely to the beta phase. ASTM E 8 (“Standard Test Method for Tension Testing of Metallic Materials”). This method covers the tension testing of metallic materials in any form at room temperature-specifically the methods of determination of yield strength, yield point, tensile strength, elongation, and reduction of area. The tension tests determine the strength and ductility of materials under uniaxial tensile stress.

Mechanical properties of the produced bars from the evaluated alloys are shown in Table 2.

TABLE 1 Element Content (wt %) Example Al Mo V Fe Ti 1 1.5 4.5 1.0 1.5 balance 2 2.5 5.5 2.0 2.5 balance 3 3.0 6.5 3.0 3.5 balance 4 3.5 8.0 3.5 3.8 balance

TABLE 2 Mechanical Properties Yield Ultimate Strength Elongation Reduction Example Strength σ_(B), MPa σ_(0.2), MPa δ, % of Area, Ψ % 1 1100 1020 25.1 58.7 2 1250 1190 20.2 46.4 3 1440 1390 6.1 10.2 4 1520 1480 4.9 7.3

Commercial Practicability

The claimed metastable β-titanium alloy as compared to the known alloys has the specified optimal combination of beta and alpha stabilizing alloying elements, which ensure high strength and ductility of the as-heat treated ally. It is low cost and can be used for production of a wide range of critical parts, especially fastener components and different coil springs. 

1: Process for preparation of a metastable β-titanium alloy containing aluminum, molybdenum, and iron, and vanadium, consisting essentially of (mass %): Aluminum 1.5-3.5 Molybdenum 4.5-8.0 Vanadium 1.0-3.5 Iron 1.6-3.8 Titanium balance,

wherein the molybdenum is added as ferromolybdenum with 55-75% of molybdenum and a balance of iron. 2: The process of claim 1 wherein the vanadium is added as ferrovanadium with 65-80% of vanadium and a balance of iron. 3: The process of claim 1 wherein the vanadium is added as a Ti—Al—V system scrap. 4: A process for preparation of a metastable β-titanium-base alloy consisting of (mass %): Aluminum 1.5-3.5 Molybdenum 4.5-8.0 Vanadium 1.0-3.5 Iron 1.5-3.8

Incidental impurities; and Titanium balance, wherein the molybdenum is added as ferromolybdenum with 55-75% of molybdenum and a balance of iron. 5: The process of claim 4 wherein the vanadium is added as ferrovanadium with 65-80% of vanadium and a balance of iron. 6: The process of claim 4 wherein the vanadium is added as a Ti—Al—V system scrap. Metastable β-titanium alloy containing aluminum, molybdenum and iron, wherein it additionally contains vanadium, at the following content of components (mass %): Aluminum 1.5-3.5 Molybdenum 4.5-8.0 Vanadium 1.0-3.5 Iron 1.6-3.8 Titanium balance 